1. Field of the Invention
The present invention relates to a recording medium suitable for recording by use of ink, and, particularly, to a recording medium excellent in the ink receptivity and sharpness of recorded images, and having good light fastness and good blocking resistance.
2. Description of the Related Art
In ink jet recording processes, recording is carried out by forming ink droplets according to various ink (or recording liquid) ejection methods, for example, an electrostatic attraction method, a method in which mechanical vibration or displacement is applied to ink by use of a piezoelectric element, a method in which the pressure generated by heating ink and forcing it to foam is utilized, and so forth; and flying the droplets so that some or all of them are adhered on a recording medium such as paper. The process has been noted as a recording process that is substantially free from noise and can perform high-speed and multi-color printing.
As ink jet recording ink, those mainly comprised of water and hydrophilic solvents are used in view of safety and recording performance, and polyvalent alcohols are often added thereto to prevent nozzles from clogging and improve the ejection stability.
As recording media used in the ink jet recording process, there have been hitherto used those comprising a porous ink receiving layer provided on an ordinary paper or on a substrate called an ink jet recording paper. However, accompanying improved performance (such as increasing high-speed or multi-color recording) and widespread use of ink jet recording devices, properties in higher grade and wider range have been increasingly required also in respect of the recording media. Namely, it is necessary for a recording medium used in ink jet recording for obtaining recorded images having high resolution and high quality, to satisfy fundamental requirements that;
(1) ink is received in the recording medium as quickly as possible;
(2) even when ink dots have been overlapped, the ink adhered later may not flow into the dots adhered earlier;
(3) ink droplets may not diffuse over the recording medium to excessively enlarge the size of ink dots;
(4) the ink dots are substantially round with smooth peripheries;
(5) the ink dots have a high OD (optical density), and are free from unclearness around the dots; etc.
In addition to the performance characteristics required above the following characteristics are further required when recorded images having high resolution comparable to color photographs are to be obtained by a multi-color ink jet recording process:
(6) Coloring components of the ink have a good coloring property;
(7) the recording medium has particularly good ink fixation since the same number of droplets as the number of the colors of ink may sometimes adhere overlapping at the same portion;
(8) the surface is glossy;
(9) it has high whiteness;
(10) recorded images have high light fastness;
(11) the recording medium has high blocking resistance without any stickiness on its surface of ink-received portions; etc.
The recorded images formed by the ink jet recording process have been hitherto used solely for surface image observation, but, accompanying improved performance characteristics and widespread use of ink jet recording devices, recording media suitable for uses other than surface image observation are being sought after. The use of the recording medium for other than the surface image observation may include a case in which images formed by projecting recorded images on a screen or the like by means of optical equipment such as a slide and an OHP (over head projector) are observed, a color separation plate used when producing a positive plate for color printing, a CMF (color mosaic filter) used for color display by liquid crystals, etc.
While diffused light of recorded images is principally observed when a recording medium is used for surface image observation, what is required in the recording medium for the above uses is transmitted light of the recorded images. Thus, in addition to the characteristics required in the recording medium for the ordinary ink jet recording, the recording medium is further required to be excellent in the light transmission properties, in particular, the linear transmittance.
However, in the present state of the art, no recording medium that has satisfied all of these required characteristics has been available.
Moreover, many of the conventional recording media for the surface image observation have employed a system in which a porous ink receiving layer is provided on its surface and ink is received in porous voids in the layer to fix a recording agent. Accordingly, because of the porousness, there has been no gloss on the surface of the recording media. On the other hand, there are strong demands for a recording medium having gloss on the surface or a recording medium having good light transmission properties, as mentioned above. It is essential for such a recording medium that the ink receiving layer has a non-porous surface. However, when the ink receiving layer has a non-porous surface, there have involved problems that non-volatile components in ink, such as polyvalent alcohols, remain unremoved from the surface of the recording medium for a long time after recording to prolong the time for drying and fixing ink, whereby clothes may be soiled when coming into contact with the recorded images, or the recorded images may be damaged.
To cope with such problems, attempts have been hitherto made to form a non-porous ink receiving layer by use of a water soluble polymer having a good affinity for ink.
For example, Laid-Open Japanese Patent Application No. 44386/1985 proposes a light transmissive recording medium comprising an ink receiving layer formed on a polyester film by using polyvinyl alcohol. However, in such a recording medium, although having good linear transmittance and ink receptivity, there is a problem that hydrogen bonds between polyvinyl alcohol molecules become so strong when printing is carried out in an atmosphere of low temperature and low humidity that the ink receptivity may be extremely lowered.
U.S. Pat. No. 4,503,111 also proposes a recording medium comprising a non-porous ink receiving layer formed by mainly using polyvinyl pyrrolidone. However, such a recording medium involves a problem that the light fastness of recorded images is so poor or the stickiness of the surface of ink receiving layer having received ink is so strong that blocking may occur when printed matters are overlapped each other or a paper or the like is placed on a recording face.